I have never heard garlic powder used against wireworms and slugs..my first instinct is 'I doubt it does'. Wireworms are usually controlled by soil disturbance and their number will reduce over time..and if you have any grass paths, getting rid of the grass will help too. When I have used nematodes for my spuds...or any other crops..I target the specific area/crop and not 'wasting' good stuff all over the place. Nematode treatment, when done correctly and in right weather and ground conditions have quite long lasting effect and I haven't needed to re-treat the same area following year for other kind crops..so effectively I as I rotate my crops, each year the treated area is rotated too....but I don't tend to use it routinely neither..not every year.
As for blight..well..in a allotment situation where everybody's crops are so close together..blight spores will be all over the place anyway..would it really make that much difference if the infected crops would be cleared away quickly...
I don't stress over these things and rather grow early varieties that will miss the blight season or varieties that are know to have some natural resistance..in a bad year they will get 'hit' too but perhaps that little bit later on.
